easy test
The most common method is to use a "half-silver chloride cell." Simply using a standard voltmeter by itself is useless. The cell consists of a piece of pure silver, connected by a copper wire to a sensitive voltmeter and then with another copper wire to a probe that can be used to make electrical contact with various pieces of underwater metal. The Silver/Silver Chloride (or Ag/AgCl) reference electrode is the reference electrode of choice, because it’s easily and cheaply prepared. Almost any digital voltmeter can be used to take the measurements. Analog voltmeters that can read voltages as low as 1/1000 of a volt (one millivolt, or mv) can also be used, except an analog meter will give you a very low reading (if any at all) in fresh water.To use the voltmeter to check on the adequacy of the sacrificial zinc, one lead wire is connected to the silver electrode and immersed in the water in which the boat is floating. The other wire from the voltmeter is connected to a piece of metal in the boat that is in contact with the seawater (the prop shaft, for example).Voltage Ranges vs Silver/Silver Chloride Reference ElectrodeBronze: 500 - 700 mV < 500 mV Bz is eroding - Add zinc, > 700 mV Bz overprotected - Remove zincSteel: 750 - 950 mVAluminum: 800 - 1050 mVLover milli-Voltages indicate metal erosion - ADD Zinc.Higher mill-Voltages indicate over-protection - Remove Zinc.Galvanic Series of Metals in Sea Water from the least noble to the most noble and their potential voltage (note: the table is not complete).Corrosion Potential in mVMagnesium and Magnesium Alloys -1600 to -1630Zinc: -980 to -1030Aluminum: - 760 to -1000Mild Steel: -600 to 710Copper; -300 to -570Brass: -300 to -400Lead: -190 to -25018-8 S/S Type 304: -50 to -10018-8 S/S Type 316: 0 to -100Graphite: +200 to +300In order to have proper protection, each metal in the grounding circuit should have a reading at least -200 mV below its stated potential range of corrosion. Example: a brass through hull should give a reading of no more than -500 to -600mV (-300 + -200 = - 500 mV) to have proper protection in the grounding system. Another way of looking at it is that the zinc should be absorbing the electrolysis given its corrosion potential at -980 to -1030 and will function as the anodic agent in the current flow between all the equipment connected in that series of grounding. If the voltage at the brass through-hull is -300 to -400, that means that it is functioning as a potential "sacrificial" anode.The voltage on all underwater hardware connected to the bonding system should be the same (IF NOT, there are problems in the wiring or connections)